1. Field of Invention
The present invention relates to a thin film piezoelectric element that uses a thin film piezoelectric material, a thin film piezoelectric actuator and a thin film piezoelectric sensor that use the thin film piezoelectric element, and a hard disk drive and an ink jet printer that include the thin film piezoelectric actuator.
2. Background Art
With the increasing demand for lead-free piezoelectric materials in recent years, studies on potassium sodium niobate ((K,Na)NbO3 (also referred to as KNN hereinafter)) have been actively pursued. KNN is attracting much attention since it has a relatively high Curie temperature among lead-free piezoelectric materials and offers favorable piezoelectric properties.
Meanwhile, commercialization of piezoelectric elements that use thin film piezoelectric materials instead of bulk piezoelectric materials is progressing. Examples thereof include piezoelectric sensors, such as gyro sensors, pressure sensors, pulse wave sensors, shock sensors, and microphones, that use the piezoelectric effect of converting a force applied to a piezoelectric thin film into a voltage; piezoelectric actuators, such as hard disk drive head assemblies and ink jet print heads, that use the inverse piezoelectric effect of a piezoelectric thin film deforming when a voltage is applied to the piezoelectric thin film; and other devices that similarly use the inverse piezoelectric effect, such as speakers, buzzers, and resonators.
Piezoelectric materials made into thin films enable size-reduction of elements and widen the field of application of piezoelectric materials. Moreover, since a large number of elements can be fabricated on a substrate at the same time, suitability for mass production is enhanced. There are also many advantages in terms of performance, such as improved sensitivity when thin film piezoelectric materials are used in sensors.
Patent Literature 1 describes that a piezoelectric thin film composed of a perovskite-type or tungsten-bronze-type material and having a purely c-plane orientation can be formed by forming the piezoelectric thin film on an oxide thin film on a single crystal silicon substrate, and that piezoelectric properties optimum for devices can thus be obtained.
Patent Literature 2 describes a potassium sodium niobate thin film (KNN thin film) that achieves a stable, high piezoelectric property −d31, in which the majority of crystal grains constituting the piezoelectric thin film have a columnar structure that is longer in the thickness direction than in the substrate surface direction and the average crystal grain size in the surface direction is 0.1 μm or more and 1 μm or less.
Patent Literature 3 describes that a dielectric thin film formed by a MOCVD method is annealed in an oxidizing gas atmosphere containing ozone to decrease the number of defects in the network structure of the dielectric thin film and that the leakage current is reduced as a result.
Non Patent Literature 1 and Non Patent Literature 2 describe that adding Mn to a potassium sodium niobate thin film (KNN thin film) decreases the hole density and the number of oxygen vacancies and that the leakage current properties are improved as a result.
[Patent Literature 1] Japanese Unexamined Patent Application Publication No. 09-110592
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2008-159807
[Patent Literature 3] Japanese Unexamined Patent Application Publication No. 10-182300
[Non Patent Literature 1] Lee et al: Current Applied Physics 11 (2011) S266
[Non Patent Literature 2] Wang et al: Applied Physics Letters 97, 072902 (2010)